Enable building of `foreground default` version about unit

[thresheek]

While it's true wrt docker-based environments, that argument does not hold up to systemd - it happily accepts Type=forking to look after forking software.

[igorsysoev]

The program is called unitd which means "unit daemon".
It would be strange if its default mode is foreground.
It can probably be renamed to something else like "unit service".
The name "unit" however is reserved for unit control utility.

[VBart]

Adding a compile time option only complicates things.

Most users will never compile Unit, they will install it from packages. Currently, the behaviour is straightforward: daemonized by default and an option to avoid daemonization. If we introduce the compile time option, users won't know the default.

[xytis]

@VBart I agree that compile-time flag which I initially proposed would introduce mixup. But as I understood from projects roadmap, you are aiming to bring Nginx Unit into container world as a first-class citizen, for me it just makes sense that applications would start in foreground unless specifically instructed not to do so. For example, with a -d|--daemon flag.

But @igorsysoev has a point too. Which operating systems do have that utility? I searched Debian jessie|stretch|buster and CentOS7 for binary matching that name, yet I could not find any.

[igorsysoev]

I meant that "unit" name is reserved for our future control utility.
The utility will control local or remote unitd in the same way as docker control dockerd.

[xytis]

Oh, thanks for the clarification.
Hashicorp has a similar approach with using single binary and multiple "actions". (unit daemon vs unit [command])

But I see the problem with naming here. I guess I will just have to accept that unitd is daemon by definition :)